Buying an unlocked iPhone 5
It looks like the US Apple Store won't get unlocked iPhone 5's at launch, and if the same pattern as previous devices holds true, it might be a month or more before they are available.
In contrast, the UK will have unlocked devices one day one.
In the past, I've just bought UK models and then used them both in the UK and the US.
However, this time this plan might not work so well - there are three distinct models of iPhone 5. One is CDMA (which we can discount, as it's not used in the UK), and two GSM versions. The difference between GSM models seems to be the LTE channels in use.
1. GSM model A1428*: UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 4 and 17)
2. GSM model A1429*: UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 1, 3, 5)
Now, from what I can tell, there are different LTE bands used in the UK and US, and so a UK iPhone 5 bought in the UK might not work on LTE in the US, and vice versa.
I'd love to get some confirmation of this.